1. Field of the Invention
The present invention relates to tube type heat exchangers. More particularly, the present invention relates to a process for cleaning tube type heat exchangers.
2. Description of the Prior Art
In the manufacture of heat exchangers, especially those of the shell and tube type wherein the interior of the shell houses a plurality of tubes whose ends are mounted to a tube sheet that closes the end of the shell, it is necessary that one of the final stages of the fabrication process include that of cleaning the interior of the assembled tubes. This need arises from the fact that during the various steps of the fabrication process the deposition of dirt, metal chips and other, sediment in the tubes is inescapable. Moreover, prior to completion of the assembly, it must be heat treated which results in the generation of metal oxides within the tubes. To remove these oxides, the tubes are subjected to acid pickling and thereafter it is necessary to clean the tubes to insure that no acid residue remains in the tubes upon completion of the pickling step since the presence of such acid would ultimately result in contamination of the fluid passed through the tubes during operation of the heat exchanger.
In the past, it has been the practice to clean the tubes of such heat exchangers by manually driving a swab attached to a wire or the like through the tube. This is both a laborious and time consuming procedure especially when it is considered that it is not uncommon for heat exchangers of this type to include thousands of tubes.
U.S. Pat. No. 3,631,555 to Linz et al teaches an apparatus operable to propel a cleaning pellet by means of compressed air or other motive fluid through the interior of the tubes assembled in the tube sheet of a heat exchanger or other similar equipment.
The efficiency of a heat exchanger of the shelltube type is unavoidably lessened after some time of operation due to deposits on the tube walls, especially to deposits along the inner tube walls. Such deposits may be caused by mechanical impurities carried by the media flowing through the tubes which condense along the tube walls or by substances contained in the media in a state of solution but precipitated therefrom by thermal and/or chemical influences. These deposits impede the heat transition to transfer through the tube walls and thereby deteriorate the efficiency of the heat exchanger. When this efficiency is lowered to a certain fraction of the original efficiency thereof, the tubes have to be cleaned mechanically and/or chemically to restore the original efficiency.
U.S. Pat. No. 4,237,962 to Vandenhoeck teaches a particulate cleaning medium introduced between the inlet ends of the tubes and the tube sheet and is then forced in a direction counter to the direction of flow of the first fluid through the tubes along the exterior surfaces of the tubes to the inlet ends of the tubes so that the particulate cleaning matter is introduced into the tubes and is directed against the inner walls of the tubes as the direction of flow is changed so that the particulate cleaning media flows through the tubes in the direction of the flow of the first fluid.
Many methods and apparatus are in use for removing impurities and other noxious substances from the medium passing through the pipes or tubes and for periodically cleaning these tubes. For instance, chlorine is added to the fresh cooling water for precipitating the above-named organic substances entering into the tubes. Or, in the alternative, mechanical impurities are removed by filtering the fresh water. Furthermore, in circulatory cooling systems the increased hardness of the circulating cooling water due to evaporation is counteracted by chemically softening the water. As a rule, the pipes or tubes of the tube-type heat exchangers are only periodically cleaned by mechanically and/or chemically removing the above-named deposits from the tube walls.
Loose sludge may be removed by increasing the velocity of the cooling water, by heat exchanger rinsers and the like, solid sludge is removed by ordinary wire brushes, while very hard sludge deposits are drilled out, and solid stone, such as lime, deposits are dissolved chemically.
Due to the fact that each subsequent cleaning of the heat exchanger can only be effected after a certain finite period of time, the level of average heat transfer of the cooling tubes, or of the heat exchanger efficiency is, in many cases considerably, lower than the maximum values obtained immediately after the cleaning. For reasons connected with the particular operation of the particular plant the operating period of the heat exchanger ascertained as being economical sometimes has to be exceeded, the average vacuum of the heat exchanger being further impaired as a necessary consequence thereof.
U.S. Pat. No. 3,021,117 to Taprogge teaches an apparatus for self-cleaning vacuum heat exchangers.
Pipeline efficiency and volume can be lost by scale build up in the interior lining of the pipe. Mechanical pigs and/or gelled chemical pigs have been used to remove the scale. The mechanical pigs ar normally solid bullet-shaped devices which have wire brushes or abrasive surfaces to physically abrade the scale interior of the pipe. The gelled chemical pigs, on the other hand, remove the surface deposits by dissolution and/or by picking up loose debris as they pass through the pipeline.
U.S. Pat. No. 4,543,131 to Purinton, Jr. teaches a method of cleaning the interior of pipelines. The method includes passing an aqueous gelled pig containing an aqueous, cross-linked gelled galactomannan gum, or derivative, through the pipeline.
U.S. Pat. No. 4,216,026 to Scott teaches a method for cleaning pipelines using an aqueous gel in which plugs of Bingham plastic fluids are effective in picking up loose debris and minor amounts of liquids as the plug moves through the pipeline. The plug is used in combination with mechanical scrapers.
U.S. Pat. No. 4,003,393 to Jagger et al. also teaches a method of removing fluids and solids from a pipeline using an organic liquid gel with a metal salt of an aliphatic ester or orthophosphoric acid.
While the aforementioned aqueous gels have many desirable properties, certain types of scale or scale components are effectively removed only by an organic solvent. In most instances, a "fill and soak" type treatment with a liquid solvent is not practical due to th volume of solvent required. Waste disposal of such a large volume of material is also a commercial problem.
There are many organic gels described in the literature. For example, U.S. Pat. No. 3,505,374 to Monroe teaches the use of magnetite salts of alkyl oleyl orthophosphate as gelling agent for hydrocarbons and halogenated hydrocarbon liquids. U.S. Pat. No. 3,757,864 to Crawford et al. teaches that the pressure drop of a confined non-polar organic liquid in motion due to friction is lessened by admixing with the liquid one or more aluminum salts of an aliphatic orthophosphate ester. U.S. Pat. No. 3,757,864 to Crawford et al. also teaches that such esters can gel the liquids. U.S. Pat. No. 3,219,619 to Dickerson teaches thickened hydrocarbons with t-butylstyrene interpolymers containing metal carboxylate groups. U.S. Pat. No. 3,527,582 to Haigh et al. teaches reversible gels of liquid hydrocarbons using a crosslinked latex polymer of an alkyl styrene. But, as U.S. Pat. No. 3,505,374 to Monroe teaches, thickened organic fluids are not the same as organic gels.
With organic gels, the gel consistency will not disappear on dissolution of the gel. With sufficient dissolution, the solvent swollen gelling agent will appear as a distinct phase in suspension. Moreover, the gel structure has a viscosity profile that is quite different from liquids that are merely thickened but not gelled.
If a gel is to be used as a pipeline pig, the rheology and chemical and physical properties of the gel must meet certain demands. For example, the gel must be viscoelastic and self-sustaining so that it will not break up as it is being forced through the line under pressure. It is also desirable for the gel to have the capacity to retain suspended solids and the ability to sustain a gel/liquid interface. This later capability is needed because in many instances it is desirable to displace with the gelled pig and/or to drive the pig directly with a liquid under pressure. Also, it is desirable in many instances to use a pig train which will have one or more chemical pig segments and the gel desirably would have a gel structure that would prohibit or substantially inhibit comingling of liquids in front of and/or behind the gelled pig (sometimes called fluid by-pass).
It has now been discovered that organic gels that include: (a) a non-polar, liquid, organic solvent and (b) a gelling amount of a mixture of (1) an alkyl oleyl phosphate and (2) an alkali metal aluminate, have very desirable properties. U.S. Pat. No. 4,473,408 to Purinton, Jr. teaches these organic gels can be used as gelled pigs to remove organic soluble scale or scale contaminants from pipeline and can also be used in a variety of other ways.
U.S. Pat. No. 3,415,729 to Dana teaches a method for removing paraffin deposited on the inside of the well tubing or of the oil discharged line of oil wells.
U.S. Pat. No. 3,384,512 to Frederick teaches a pigging device launching detecting system. Means are provided for launching a pigging device into a carrying line. An electrical sensing means is provided for responding to the passage of a magnet-containing pigging device past a predetermined point in the pipeline. Control means are operable in response to signals from the electrical sensing means and are adapted to regulate the launching means.
U.S. Pat. No. 3,209,771 teaches the use of gelled bodies for separating two fluids flowing in a pipeline. U.S. Pat. No. 3,225,787 teaches an attempt to improve the technique of U.S. Pat. No. 3,209,771 by employing an elongated gel filled pipeline pig having elastic reinforced rubber sidewalls and thickened ends. The latter technique was employed to overcome the problem of the gelled body of U.S. Pat. No. 3,209,771 breaking down in long pipelines. However while solving this problem several new problems ensued. First, due to the thick walls of the pig taught in U.S. Pat. No. 3,225,787 the pig lost some of its flexibility and tended to be blocked by "stalactites" located at welded joints in the line. Furthermore, the pig could only be employed in one size pipeline. Canadian Patent No. 903,621 teaches a device to overcome the blocking problem by employing an elongated gel-filled pipeline having thin lateral walls and elastic end walls. The walls are sufficiently thin so that they are ripped by stalactites and flow on without substantial pressure build-up.
An ideal pipeline pig would be a gelled self-sustaining mass which does not break up in line pipelines and which can be readily converted to a liquid for disposal at the end of the flow cycle. Furthermore, it would be preferable if the pig could change size so that it could flow through different size conduits.
U.S. Pat. No. 4,003,393 to Jaggard et al. teaches a gel-like mass which does not break up in long pipelines and which can readily be returned to a liquid form at the end of the use cycle. In addition, the pig can be flowed directly from one size pipe to another. Also, the gelled pig can be employed as a wiper plug to remove various fluids (e.g. hydrocarbons, asphaltines, paraffins), solids and semi-solids such as sand, tar, corrosion products and the like from conduits. The gel not only wipes surfaces clean but can absorb a substantial amount of water without breaking down.
U.S. Pat. No. 3,565,689 to Lowe et al. teaches a source of dry pressured gas applied about a rear end surface of an elongated projectile in a confined space to propel the projectile into the interior of a tube to be purged of liquid and liquid vapor. The supply of gas is maintained under pressure about the rear end surface of the projectile to drive it toward a remote open end of the tube.
U.S. Pat. No. 4,440,194 to Kinumoto et al. teaches moving bodies for performing work in the interior of pipes for transporting town gas, petroleum, water and like fluids, and to a method of performing work within pipes with use of such a body.
As shown, supra, numerous innovations for cleaning pipes have been provided in the prior art that are adapted to be used to accomplish work in the performance of specific individual operations. While these innovations may be suitable for the specific individual purposes to which they address, they would not be suitable for the purposes of the present invention as heretofore described.